The present invention relates to the use of hydroxyalkylphenones as initiators for radiation curing of aqueous prepolymer dispersions.
Photochemically initiated polymerization reactions have attained great importance in industry, especially when the object is rapid curing of thin layers, such as, for example, in the curing of varnish and resin coatings on paper, metal and plastic. In this context, radiation curing in the presence of photoinitiators is distinguished, as compared to conventional methods for curing coatings, by savings in raw materials and energy, low thermal stress on the substrate and, in particular, a high rate of curing. In addition, more recently, interest has increased in aqueous systems free of volatile components as environmentally acceptable coating agents, especially because, using these, pollution of the environment by organic solvents and monomers can be avoided or at least reduced; the danger of fire when handling coating, impregnating and adhesives formulations can be reduced; and the high costs of organic solvents can be saved.
However, aqueous coating agents of this type make particular demands on the photoinitiator used. Particularly relevant are the solubility of the initiator in the reaction mixture, the stability on storage in the dark of the reaction system to which the initiator has been added, the chemical stability of the initiator in aqueous systems and its behavior when the water is removed from these aqueous systems and, above all, on its reactivity, which determines the rate of curing.
Photoinitiators which are suitable for use in aqueous prepolymer dispersions curable by radiation and which have hitherto been mentioned are mainly benzophenone derivatives, benzoin ethers, benzil monoketals, anthraquinones, thioxanthones and their derivatives and other aromatic ketones. However, a number of disadvantages are associated with these photoinitiators. These include, inter alia, insufficient stability of the systems curable by radiation to which these initiators have been added and yellowing of the cured polymers due to residues of the initiator or decomposition products arising therefrom during the photochemical reaction. In particular, however, these photoinitiators have an inadequate reactivity in the radiation curing of these special systems, and this makes relatively long curing times necessary.
Moreover, 1-phenyl-2-hydroxy-2-methyl-1-propanone, belonging to the group of hydroxyalkylphenones, has already been recommended as a photoinitiator for special aqueous prepolymer systems curable by radiation (compare European Offenlegungsschrift No. 33,896). This compound, which has been found to be particularly suitable as an initiator in non-aqueous photopolymerizable systems and has significant advantages compared to other initiators, such as good ability to be incorporated in the photopolymerizable systems, good stability on storage in the dark and chemical stability and an increased photoinitiating effectiveness, has, however, disadvantages in the radiation curing of aqueous coating agents. Thus, it must be employed in relatively large amounts in aqueous systems, in particular for the curing of thin layers, and furthermore it has only inadequate reactivity in initiating radiation curing in these special prepolymer systems curable by radiation. This makes relatively long curing times necessary and leads to non-optimum utilization of industrial irradiation equipment.